Single-stage, single-switch, islolated power-supply technique with input-current shaping and fast output-voltage regulation

ABSTRACT

A new single-stage, single-switch, input-current-shaping technique which combines the boost-like input-current shaper with a continuous-conduction-mode dc/dc output stage is described. Due to the ability to keep a relatively low voltage (&lt;450 Vdc) on the energy-storage capacitor, this technique is suitable for the universal line-voltage applications. The voltage on the energy-storage capacitor is kept within the desirable range by the addition of two transformer windings. One winding appears in series with the boost inductor during the on time, whereas the other winding appears in series with the same inductor during the off time. By connecting the windings so that the voltages across them when they conduct the inductor current are in opposition to the input voltage, the volt-second balance of the boost-inductor core is achieved at a substantially lower voltage of the energy-storage capacitor compared to the other known approaches. In addition, for the forward-converter-type single-stage, single-switch input-current shapers, a direct transfer of a part of the input energy is achieved by the winding which appears in series with the boost inductor during the on time, while for the flyback implementation a direct transfer of a part of the input energy is achieved by the winding which appears in series with the boost inductor during the off time.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 08/669,001 filed June 21, 1996 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved single-stage, single-switch, isolated power-supply technique with input-current shaping and fast output-voltage regulation and, more particularly, to circuit topology which reduces the maximum voltage on the energy- storage capacitor.

2. Description of the Prior Art

A number of standards regulate the harmonic content of the line current drawn from the ac mains by a piece of electronic equipment. The specific requirements of these standards depend on the type of the equipment and its power level (see, Electromagnetic Compatibility (EMC) - Part 3: Limits - Section 2: Limits for harmonic current emissions (equipment input current ≦16 A per phase), IEC 1000-3-2 Document, First Edition, 1995). To comply with these standards, input-current shaping (ICS) of off-line power supplies is necessary. So far, a variety of passive and active ICS techniques have been proposed in the prior art. While passive techniques can be the best choice in many cost-sensitive applications, the active ICS techniques are used in the majority of applications due to their superior performance.

The most commonly used active approach, which meets high power-quality requirements, is a "two-stage" approach (see L. H. Dixon, Jr., "High power factor preregulators for off-line power supplies," Unitrode Switching Regulated Power Supply Design Seminar Manual, Paper I2, SEM-700, 1990). In this approach, a non-isolated boost-like converter, which is controlled so that the rectified line current follows the rectified line voltage, is used as the input stage that creates an intermediate dc bus with a relatively large second-harmonic ripple. This first ICS stage is then followed by a dc/dc converter which provides isolation and high-bandwidth voltage regulation. For high-power levels, the ICS stage is operated in the continuous-conduction mode (CCM), while the discontinuous-conduction-mode (DCM) operation is commonly used at lower power levels due to a simpler control.

Although relatively simple, mature, and viable in wide power-range applications, the two-stage approach suffers from several drawbacks. First, due to two-stage power processing, conversion efficiency is reduced. Second, a separate ICS stage adds components and complexity and, consequently, increases the cost. The cost increase is especially undesirable for low-power supplies used in consumer electronic products such as, for example, personal computers, low-end printers, home appliances, etc.

In an effort to reduce the component count and also improve the performance, a number of "single-stage" ICS techniques have been introduced recently (see M. Madigan, R. Erickson, E. Ismail, "Integrated high-quality rectifier-regulators," IEEE Power Electronics Specialists Conf. (PESC) Record, pp. 1043-1051, June 1992; L. D. Stevanovic, S. Cuk,. "Input current shaping and regulation of multiple outputs in a single isolated converter," IEEE International Telecommunication Energy Conf. (INTELEC) Proc., pp. 326-333, September 1993; M. Brkovic, S. Cuk, "Novel Single Stage AC-to-DC Converters with Magnetic Amplifiers and High Power Factor," IEEE Applied Power Electronics Conference (APEC) Proc., pp. -453, March 1995; R. Redl, L. Balogh, "Design Considerations for Single-Stage Isolated Power-Factor-Corrected Power Supplies with Fast Regulation of the Output Voltage," IEEE Applied Power Electronics Conference (APEC) Proc., pp. 454-458, March 1995; and H. Watanabe, Y. Kobayashi, Y. Sekine, M. Morikawa, T. Ishii, "The Suppressing Harmonic Currents, MS (Magnetic-Switch) Power Supply," IEEE International Telecommunication Energy Conf. (INTELEC) Proc., pp. 783-790, October 1995). In a single-stage approach, input-current shaping, isolation, and high-bandwidth control are performed in a single step, i.e., without creating an intermediate dc bus. Generally, these converters use an internal energy-storage capacitor to handle the differences between the varying instantaneous input power and a constant output power.

Among the single-step circuits, a number of prior art circuits described in Madigan, Brkovic and Watanabe and shown in FIGS. 1 through 3 seem particularly attractive because they can be implemented with only one semiconductor switch and a simple control. FIGS. 1 and 3 show the forward-converter implementations of the single-stage, single-switch (S⁴) ICS concepts described in Madigan and Watanabe, while FIG. 2 shows the Cuk-converter implementation of the idea introduced in Brkovic. Except for the circuit in FIG. 3, all other S⁴ ICS circuits employ the DCM boost converter in the ICS stage. In fact, in these circuits, low input-current harmonic distortions are achieved through the inherent property of the DCM boost converter to draw a near sinusoidal current if its duty cycle during a line period is held relatively constant.

While boost inductor L₁ in the ICS stage of the converter in FIG. 1 must operate in DCM (i.e., during a part of a switching cycle the current in L₁ has to be zero), output inductor L₂ can be designed to operate either in DCM or CCM. According to the analysis in Redl, if L₂ operates in CCM, the voltage on energy-storage capacitor C_(B) shows a strong dependence on the line voltage and output current. In fact, the energy-storage-capacitor voltage increases as the rms of the line voltage increases and/or output current decreases. When the load current is decreased while L₂ operates in CCM, the duty cycle will not change in direct response to the change of the current. Because initially the duty cycle stays constant, the input source will add more charge to energy-storage capacitor C_(B) than the charge withdrawn from the capacitor by the load. As a result, the energy-storage capacitor voltage will increase, forcing the voltage control loop to decrease the duty cycle so that a new charge balance is established at a higher energy-storage-capacitor voltage. As explained in Redl, for a converter in FIG. 1 designed for universal line-voltage range from 90 Vac to 270 Vac, the voltage on C_(B) can exceed 1000 Vdc at high line voltage and light load current if L₂ operates in CCM. As a result, the approach proposed in Madigan is not practical for applications with the universal line-voltage range. The voltage stress on the energy-storage capacitor can be substantially reduced by employing the variable switching-frequency CVSF) control as described in M. M. Jovanovic, D.M.C. Tsang, F. C. Lee, "Reduction of Voltage Stress in Integrated High-Quality Rectifiers-Regulators by Variable-Frequency Control," IEEE Applied Power Electronics Conference (APEC) Proc., pp. 569-575, March 1994. However, even with a wide-range of switching frequency, the energy-storage capacitor voltage cannot be kept below 500 Vdc. As a result, energy-storage capacitors with voltage ratings above 600 Vdc must be used. This makes the circuit in FIG. 1 and any other S⁴ ICS circuit using the same approach impractical because high-voltage-rated storage capacitors are more expensive and bulky compared to desirable 450-V-rated capacitors which are typically used in conventional ICSs. The voltage on C_(B) in the circuit in FIG. 1 can be kept below 450 Vdc only if output inductor L₂ is designed to operate in DCM. As explained in Redl, in that case the voltage on C_(B) is independent of the load current. However, for low-voltage, high-current applications, the DCM operation of L₂ is not desirable because it results in much higher stresses in semiconductor components compared to the CCM operation.

The circuit in FIG. 2 can also operate with inductor L₂ in CCM. Because Magnetic Amplifier (MagAmp) MA1 operates as a switch, the output-voltage control and energy-storage capacitor voltage control can be done with two independent loops. Voltage across energy-storage capacitor C_(str) in this circuit is kept within the desired voltage range (<450 Vdc) by a slow MagAmp regulation loop, while the semiconductor switch is used in the fast output-voltage regulation loop. In addition, another fast control loop is used to shape the input current. The major drawbacks of this approach are the complexity of control and the need for a relatively expensive square-loop magnetic material for the implementation of the MagAmp. In addition, the energy transfer in the circuit in FIG. 2 is done through capacitor C₃, which is a less desirable energy-transfer mechanism than the energy transfer through a rectifier.

The circuit in FIG. 3 employs the Magnetic-Switch (MS) concept to achieve ICS with CCM operation of both ICS inductor L₁ and output-filter inductor L,. The MS is implemented with an additional winding of the transformer so that its voltage is proportional to the primary-winding voltage. The MS winding is connected in the circuit with the polarity in opposition to the energy-storage capacitor voltage when the switch is on. As a result, when the switch is on, a substantial voltage is applied to the series combination of C₁ and choke L₁ forcing the flow of the input current. When the switch is off, the L₁ energy is discharged to capacitor C_(in) through diode D₁, while the transformer is reset through the resonance of its magnetizing inductance L_(m) and capacitor C₁. Since the MS is not independently controllable, but it is controlled by the primary switch, this circuit suffers from a relatively high voltage on C_(in) even when a VSF control is employed, as reported in Watanabe.

Finally, it should be noted that in the forward-converter S⁴ ICSs in FIGS. 1 and 3 no energy is directly transferred from the input to the output. In fact, during the switch on time, the input energy is stored in the boost choke first, and then transferred to the energy-storage capacitor during the subsequent off time of the switch. The energy transferred to the output during the on time of the switch is the energy stored in the capacitor. The efficiency of the operation would be improved if a part of the energy is directly transferred to the output.

SUMMARY OF THE INVENTION

An object of this invention is a new S⁴ ICS technique which combines the boost-like ICS with a CCM dc/dc output stage coupled with an ability to keep a relatively low voltage (<450 Vdc) on the energy-storage capacitor, making this technique suitable for the universal line-voltage applications.

Briefly, this invention contemplates the provision of a single-stage, single-switch, isolated power-supply technique with input-current shaping and fast output-voltage regulation in which the voltage on the energy-storage capacitor is kept within a desirable range by the addition of two transformer windings. One winding appears in series with the boost inductor during the on time, whereas the other winding appears in series with the same inductor during the off time. These two additional transformer windings are connected so that the voltages across them, when they conduct the (boost) inductor current, are in opposition to the input voltage, so that the volt-second balance of the boost-inductor core is achieved at a substantially lower voltage of the energy-storage capacitor compared to the other known prior art approaches. In addition, for the forward-converter-type S⁴ ICSs, a direct transfer of a part of the input energy is achieved by the winding which appears in series with the boost inductor during the on time. Likewise, for the flyback-type S⁴ ICSs, a direct transfer of a part of the input energy is achieved by the winding which appears in series with the boost inductor during the off time.

Although, in the next section, the invention is described for the forward-converter implementation, this technique can be applied to any other single-ended, single-switch, isolated, single- or multiple-output topology such as the flyback, Cuk, sepic, zeta, and other converters. Furthermore, the concept described in this invention can be extended to "hard-switched" and "soft-switched" multi-switch converters such as two-switch forward and flyback converters, as well as the bridge-type topologies.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a prior art forward-converter implementation of the S⁴ ICS.

FIG. 2 is a schematic drawing of a prior art Cuk-converter implementation of the S⁴ ICS.

FIG. 3 is a schematic drawing of a prior art forward-converter implementation of the S⁴ ICS.

FIG. 4 is a schematic drawing of a specific embodiment of an S⁴ ICS forward converter proposed in this invention.

FIG. 5a-c shows the topological stages of the S⁴ ICS forward converter withL_(B) operating in DCM shown in FIG. 4.

FIG. 6 shows the key waveforms of the S⁴ ICS forward converter withL_(B) operating in DCM shown in FIG. 4.

FIG. 7 shows the line voltage and current waveforms of the S⁴ ICS forward converter with L_(B) operating in DCM shown in FIG. 4.

FIG. 8a-e shows the topological stages of the S⁴ ICS forward converter with L_(B) operating in CCM.

FIG. 9a-g shows the key waveforms of the S-ICS forward converter with L_(B) operating in CCM.

FIG. 10 shows the line voltage and current waveforms of the S⁴ ICS forward converter with L_(B) operating in CCM.

FIG. 11 is a circuit diagram of a flyback implementation of the S⁴ ICS using the concept described in this invention.

FIG. 12a and b shows oscillograms of line voltage and current of DCM implementation of experimental converter.

FIG. 13a and b shows oscillograms of line voltage and current of CCM implementation of experimental converter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows a circuit diagram of an S⁴ ICS in accordance with this invention which combines a boost-converter input stage with a CCM forward-converter output stage. In FIG. 4, the core of transformer T₁ is reset by reset winding N_(R). Transformer windings N₁ and N₂ are used to reduce the voltage of energy-storage capacitor C_(B). In addition, winding N₁ is utilized for the direct energy transfer from the input to the output. Inductances L_(1k1) and L_(1k2) represent the leakage inductances of windings N₁ and N₂, respectively. A small input capacitor C_(in) is used to filter out the switching-frequency ripple of the ICS-inductor current. Consequently, the rectified line current is the average of the current flowing from ICS inductor L_(B).

One side (node X) of boost inductor L_(B) is coupled to full-wave rectified input v_(in) ^(rec). Winding N₂ of isolation transformer T₁ is coupled, via diode D₂, between the other side (node Y) of boost inductor L_(B) and one side of primary winding N_(P) of isolation transformer T₁. Winding N₁ of isolation transformer T₁ is coupled, via diode D₁, between node Y and the other side of primary winding N_(P). The control circuit, as in prior art S⁴ ICS circuits, controls the opening and closing of a semiconductor switch SW in order to maintain a desired output voltage V_(o) across load R_(L). Output inductor L_(B) and diode D_(F) couple secondary winding N_(S) of transformer T₁ to load R_(L). Filter capacitor C_(F) is connected across load R_(L). Free wheeling diode D_(FW) provides a conduction path for output inductor L_(F) when the switch SW is off.

In operation, when switch SW is closed, the polarity of winding N₁ is such that the induced voltage across N₁ is in opposition to rectified input voltage v_(in) ^(rec). This reduces the volt-second product across boost inductor L_(B) during on time (i.e. switch closed) of switch SW, and hence reduces the energy stored in boost inductor L_(B) compared to a corresponding circuit without winding N₁. This in turn reduces the voltage of energy storage capacitor C_(B) needed to reset the core of boost inductor L_(B) during the switch off time. In addition, it should be noted that during the switch on time, winding N₁ couples input power to the load. Transformer winding N₂ is in series between the boost inductor L_(B) and the energy storage capacitor when switch SW is open. The polarity of N₂ is such, that during the switch open time, N₂ 's induced voltage opposes rectified input voltage v_(in) ^(rec) and aids the reset of L_(B), thus further reducing voltage V_(B) across the energy storage capacitor necessary to reset boost inductor L_(B).

To simplify the analysis, it is assumed that all semiconductor components are ideal. According to this assumption, the primary switch and the rectifiers do not have parasitic capacitances and represent ideal short and open circuits in their on and off states, respectively. Finally, in the following analysis, the input voltage of the converter is considered constant during a switching cycle because the switching frequency is much higher than the line frequency.

To further simplify the explanation of the circuit operation, it is assumed that the inductance of ICS inductor L_(B) in FIG. 4 is small so that inductor L_(B) operates in DCM, and that the inductance of output-filter inductor L_(F) is large enough so, that inductor L_(F) operates in CCM. Since in the DCM operation, the leakage inductances of the auxiliary windings N₁ and N₂ have no effect on the circuits operation, they are neglected in the following analysis by assuming that L_(1k1) =L_(1k2) = ). To facilitate the analysis of operation, FIGS. 5 and 6 stages of the converter during a switching cycle and its key waveforms, respectively.

Due to the DCM operation of L_(B) and the CCM operation of L_(F), at the moment immediately before primary switch SW is turned on, current i_(LB) is zero, while current i_(LF) is freewheeling through freewheeling diode D_(FW), inductor L_(F), and the output. When switch SW is closed at t=T₀, current i_(LB) starts flowing through transformer winding N₁, rectifier D₁, and switch SW, as shown in FIG. 5(a). The slope of this current is determined by ##EQU1## where v_(in) ^(rec) is the instantaneous rectified line voltage for the switching cycle commencing at t=T₀, N₁ is the number of turns of auxiliary winding N₁, N_(p) is the number of turns of the primary winding, and V_(B) is the energy-storage (bulk) capacitor voltage.

Also, at t=T₀, output-filter inductor current i_(LF) is commutated from freewheeling diode D_(FW) to forward diode D_(F) because of the induced positive secondary voltage. The rate of the increase of current i_(LF) =i_(S) is given by ##EQU2## where N_(S) is the number of secondary turns.

Finally, during the on time of the switch, transformer magnetizing current i_(M) increases according to ##EQU3## where L_(M) is the magnetizing inductance.

From FIG. 5(a), during the on time, switch current i_(SW) is given by the sum of ICS-inductor current i_(LB), primary-winding current i_(P), and magnetizing current i_(M), i.e.,

    i.sub.SW =i.sub.LB +ip+i.sub.M.                            (4)

To help visualize the components of switch current i_(SW), FIG. 6 uses different hatching patterns for each component.

Also, from FIG. 5(a) and Ampere's law, the currents flowing in the transformer are related as

    Npip+N.sub.1 i.sub.LB -N.sub.s i.sub.s =i.sub.M            (5)

By neglecting magnetizing current i_(M) in Eq. 5 because it is much smaller compared to the other currents, secondary current i_(s) during on time can be expressed as ##EQU4## From Eq. 6, it can be seen that the secondary current, which during on time supplies output energy, is composed of two components which obtain energy from different sources. The energy transferred to the secondary which is associated with primary current i_(P) is obtained from the discharging energy-storage capacitor C_(B), while the energy associated with ICS inductor current i_(LB) is drawn directly from the input line. The hatched areas in the i_(LF) waveform in FIG. 6 indicate the two current components.

When primary switch SW is turned off at t=T₁, primary current i_(P) and, consequently, secondary current i_(s) cease to flow. As a result, i_(LF) is commutated into freewheeling rectifier D_(FW), as shown in FIG. 5(b). Due to the negative voltage V_(o) applied across L_(F), i_(LF) decreases during this interval. Also, at t=T₁, current i_(LB), which was flowing through switch SW, is diverted to energy-storage capacitor C_(B), as indicated in FIG. 5(b). The downslope of i_(LB) is given by ##EQU5## Since during off time, i_(LB) needs to decrease to zero to completely reset the L_(B) core, the voltage applied across L_(B) must be negative, i.e.,

    (1+N.sub.2 /N.sub.R)V.sub.B >v.sub.in.sup.rec              (8)

From Eq. 7, it can be seen that with winding N₂ the required reset voltage for L_(B) can be obtained with a smaller V_(B) because of induced voltage (N₂ /N_(R))V_(B) across winding N₂.

The reset of the transformer core is done by reset winding N_(R). Namely, after switch SW is turned off, the magnetizing current is transferred to this winding. In addition, the reset winding also carries reflected input current i_(LB) because of the magnetic coupling between windings N₂ and N_(R). According to Ampere's law, reset winding current is given by ##EQU6## As it can be seen from FIG. 5(b), during the off time, energy stored in L_(B) is discharged to C_(B) through two paths. One path is the direct path through rectifier D₂, whereas the other path is the indirect path through the reset winding. The ratio of the directly and indirectly discharged energy is determined by turns ratio N₂ /N_(R). In FIG. 6, the energy stored in L_(B) completely discharges at t=T₂.

Finally, during the reset of the transformer, the voltage stress on switch SW is ##EQU7## When the flux in the core of the transformer is completely reset at t=T₃, the voltage across the transformer collapses to zero and reflected magnetizing current (N_(P) /N_(s))i_(M) starts flowing through the secondary, as indicated in FIG. 5(c). The topological stage in FIG. 5(c) lasts until the initiation of the next switching cycle.

It should be noted that in FIG. 6 the energy stored in L_(B) is discharged (i_(LB) falls to zero at t=T₂) before the reset of the transformer is completed at t=T₃. However, the operation of the circuit remains unchanged if the transformer reset is completed before L_(B) is completely discharged.

To ensure a proper operation of the circuit, the number of turns of windings N₁ and N₂ must be selected so that rectifier D₂ is off during the time switch SW is closed. From FIG. 4, this condition requires that ##EQU8##

To maximize the direct energy transfer, it is desirable to select ratio N₁ /N_(P) large as possible. However, a larger ratio causes larger input-current harmonic distortions, as illustrated in FIG. 7. Namely, i_(LB) current, and therefore, the input current, can not flow until the line voltage, exceeds the N₁ -winding voltage (N₁ /N_(P))V_(B). Therefore, by increasing N₁ /N_(P), the zero-crossing distortions are Increased due to a larger dead angle, i.e., a longer period with i_(LB) =0. The dead angle, θ, can be calculated from ##EQU9## where V_(in) is the rms input voltage. The relationships between θ and total harmonic distortions (THD) is discussed in M. K. Nablant, "Power Factor Calculations and Measurements," IEEE Applied Power Electronics Conference (APEC) Proc., pp. 543-552, March 1990. In addition to these crossover harmonic distortions, the input current contains the harmonic distortions caused by the finite downslope of i_(LB), as explained and quantified in K. H. Liu, Y. L. Lin, "Current Waveform Distortion in Power Factor Correction Circuits Employing Discontinuous-Mode Boost Converters," IEEE Power Electronics Specialists Conf. (PESC) Record, pp. 825-829, June 1989. Generally, these distortions decrease as the i_(LB) downslope increases. As a rule of thumb, to meet current harmonic-limit specifications (defined in the IEC 1000-3-2 document) dead angle θ should not exceed 45°.

During the time interval where the instantaneous line voltage exceeds the induced voltage on winding N₁, the peak of current i_(LB) is given by ##EQU10## where ω_(L) is the line frequency (50 or 60 Hz), ∫s is the switching frequency, and D is the duty cycle. The average of i_(LB) represents the rectified input current, as illustrated in FIG. 7.

The duty cycle of the switch is determined by the fast (wide-bandwidth) output-voltage control loop. If the voltage ripple on C_(B) is small, the duty cycle is essentially constant during a half of a line cycle. Due to the CCM operation of the output-filter inductor, duty cycle D is given by ##EQU11##

To use the above equations, energy-storage-capacitor voltage V_(B) needs to be known. By applying the input-output power balance principle to the circuit in FIG. 4, this voltage can be expressed in an implicit form as ##EQU12## where θ is the assumed efficiency of the converter and I_(o) is the output (load) current.

As it can be concluded by inspecting Eq. 16, V_(B) increases as the line voltage increases and/or the output current decreases. Therefore, V_(B) is the highest at high line and light load. Depending on the minimum load specifications, V_(B) may exceed the desired voltage level (<450 Vdc) even with the maximum possible induced voltage on winding N₂ which according to Eq. 16 reduces V_(B). In that case, the desired V_(B) can be achieved either by VSF control or by operating L_(F) in DCM at light loads. Namely, from Eq. 16, it can be seen that V_(B) is inversely proportional to ∫s. Therefore, by increasing the switching frequency as the load decreases and/or line voltage increases, the energy-storage-capacitor voltage can be limited to the desired level. One implementation of the VSF control is described in Jovanovic. At light loads, voltage V_(B) can also be li L_(F). As described in Redl, when both L_(B) and L_(F) operate in DCM, V_(B) is independent of the output current but only depends on the L_(B) /L_(F) ratio.

In the preceding explanation, it was arbitrarily assumed, for the sake of description simplifications, that the inductance of boost inductor L_(B) is small so that L_(B) always operates in DCM. However, the proposed ICS circuit shown in FIG. 4 can also properly operate for larger values of boost inductance L_(B) which result in the CCM operation of L_(B). To facilitate the explanation of the circuit operation with L_(B) operating in CCM, FIGS. 8 and 9 show the topological stages and the key waveforms, respectively. It should be noted that while the leakage inductances of the transformer have no significant effect on the operation of the circuit with L_(B) operating in DCM and, consequently, were neglected in the preceding explanation, the leakage inductances of auxiliary windings N₁ and N₂ play a major role in the operation of the circuit with L_(B) operating in CCM and cannot be neglected. As a result, in FIG. 8 the leakage inductances of windings N₁ and N₂ are shown as leakage inductance L_(1kl) in series with winding N₁ and leakage inductance L_(1k2) in series with winding N2.

Due to the CCM operation of L_(B), at the moment immediately before primary switch SW is turned on (closed), the entire boost-inductor current i_(LB) is flowing through winding N₂ and rectifier D₂ into bulk capacitor C_(B). At the same time, transformer secondary winding N_(S) is shorted because a part of the transformer magnetizing current flows through winding N_(S) and rectifier D_(F), while current i_(LF) is freewheeling through rectifier D_(FW), inductor L_(F), and the output, as shown in FIG. 8(e). After switch SW is closed at t=T₀, current i_(LB) starts commutating from winding N₂ to winding N₁, while current i_(LF) commutates from rectifier D_(FW) to rectifier D_(F). According to FIG. 8(a) which represents the circuit in FIG. 4 during the commutation interval T.sub.)-T₁ !, the i_(LB) commutation is governed by ##EQU13##

Since during the commutation interval T₀ -T₁ !, boost-inductor current i_(LB) does not change significantly due to a relative large inductance of boost inductor L_(B) required for the CCM operation, it can be assumed that

    i.sub.LB =i.sub.1 +i.sub.2 ≅const.,              (18)

i.e., ##EQU14##

From Eqs. 17 and 19, the slopes of current i₁ and i₂ during the commutation interval T₀ -T₁ ! are approximately given by ##EQU15##

The commutation of i_(LF) from D_(FW) to D_(F) can be analyzed in a similar fashion. However, this commutation is well understood and it is not important for the description of ICS properties of the analyzed circuit. Therfore, in this analysis, by neglecting the leakage inductance between primary winding N_(P) and secondary winding N_(S) of the transformer, it is assumed that this commutation is instantaneous.

Also, it should be noted that during the commutation interval T₀ -T₁ !, the voltage of the common node of the boost inductor, winding N₁, and winding N₂ (node Y in FIG. 8 (a)), is given by ##EQU16## as indicated in FIG. 9(b). The above equation was derived from Eqs. 17 and 20.

After the commutation of i_(LB) is completed at t=T₁, the entire i_(LB) flows through winding N₁ as shown in FIG. 8(b). During the T₁ -T₂ ! interval, the boost-inductor current i_(LB) is given by ##EQU17## where the approximation assumes L_(B) >L_(1k1).

From Eq.22, it can be seen that if the instantaneous rectified-line voltage v_(in) ^(rec) is smaller than the dc voltage across winding N₁, i.e., if ##EQU18## no i_(LB) can build up in inductor L_(B). As a result, the line current (average of i_(LB)) contains zero-crossing distortions caused by dead angle Θ (which can be calculated by Eq. 13) of current i_(LB).

On the other hand, for ##EQU19## i_(LB) can flow after the commutation period T₀ -T₁ ! is completed, as shown in FIG. 9(c). However, during the time intervals for which v_(in) ^(rec) is only slightly higher than winding N₁ voltage, (N₁ /N_(P))V_(B), i_(LB) is discontinuous. Therefore, during one half of a line period, the boost inductor operates in both DCM and CCM as illustrated in FIG. 10. The DCM region is more pronounced, i.e., lasts for a larger portion of a line half period, for designs with larger N₁ /N_(P) ratios.

Due to the assumed instantaneous commutation of i_(LF) from D_(FW) to D_(F) at t=T₀, i_(LF) and magnetizing current i_(M) linearly increase during the time period from t=T₀ to t=T₂, as illustrated in FIGS. 9 (d) and (e). From FIG. 8 (b), the rate of i_(LF) increase is given by ##EQU20## while the up-slope of i_(M) is given by ##EQU21##

From FIGS. 8(a) and (b), the total switch current i_(SW) during the on time is given by the sum of boost-inductor current i_(LB), primary-winding current i_(P), and magnetizing current i_(M). To help visualize the components of i_(SW), FIG. 9(f) uses different hatching patterns for each component.

Also, from FIGS. 8(a) and (b) and Ampere's law, it follows that secondary-winding current i_(S) is given by ##EQU22## during the commutation period T₀ -T₁ !, and ##EQU23## during the T₁ -T₂ ! period.

As it can be seen from Eq.28, during the T₁ -T₂ ! time interval, the output energy is supplied from bulk capacitor C_(B) by current i_(P), and directly from the source by current i_(LB).

After switch SW is turned off at t=T₂, FIG. 9, boost-inductor current i_(LB) begins commutating from winding N₁ to winding N₂, while output-filter-inductor current i_(LF) instantaneously commutates from rectifier D_(F) to rectifier D_(FW) because the leakage inductance between windings N_(P) and N_(S) is assumed zero. At the same time, the transformer core starts the reset phase by transferring magnetizing inductance i_(M) to the reset winding N_(R), thus, discharging the energy stored in the core into bulk capacitor C_(B).

From FIG. 8(c), using the assumption stated in Eq. 18, the slopes of currents i₁ and i₂ during the commutation period T₂ -T₃ ! can be calculated as ##EQU24##

Also, from FIG. 8(c), the downslope of magnetizing current i_(M) is ##EQU25## while, from FIG. 8(c) and Ampere's law, reset-winding current i_(R) during the T₂ -T₃ ! interval is given by ##EQU26##

Using Eqs. 29-31, the downslope of i_(R) during the commutation interval T₂ -T₃ ! is ##EQU27##

Since according to Eq. 12, for proper operation of the circuit N_(P) >N₁ +N₂, and for a typical design N_(R) ≈N_(P), the di_(R) /dt rate is indeed negative, as shown in FIG. 9(g).

The voltage of note Y during the commutation interval T₂ -T₃ !, V_(Y) ^(OFF), can be calculated from the circuit in FIG. 8(c), with the help of Eq. 29, as ##EQU28##

After the commutation period is completed at t=T₃, the entire boost-inductor currenti_(LB) flows through auxiliary winding N₂ into bulk capacitor C_(B), as shown in FIG. 8(d). As the transformer continues to reset after t=T₃, reset current i_(R) continues to decrease with the slope given by ##EQU29## which is obtained from Eq.3 1 by setting i₁ =0 and i₂ =i_(LB), and by using expression for di_(LB) /dt during the T₃ -T₄ ! interval given in Eq. 36. Since the slope of i_(LB) in Eq. 36 is much smaller than the slope of i₁ and i₂ in Eq. 29 due to a relative large value of L_(B), the di_(R) /dt rate during the T₃ -T₄ ! interval is much smaller than that during the commutation interval T₂ -T₃ !, as indicated in FIG. 9(g).

The transformer reset is completed at t=T₄, when reset current i_(R) becomes zero. It should be noted that when i_(R) reaches zero at t=T₄, magnetizing current i_(M) is negative and equal to ##EQU30## as shown in FIG. 9(e). Expression in Eq.35 was obtained from Eq.31 by setting i_(R) (t=T₄)=0, and recognizing from FIG. 9(c) that i₁ (t=T₄)=0, i₂ (t=T₄)=i_(LB) (t=T₄).

During the T₃ -T₄ ! interval, the downslopes of boost-inductor current i_(LB) and output-filter-inductor current i_(LF) are constant and given by ##EQU31##

The downslope of magnetizing current i_(M) during this time interval is given by Eq.30, as shown in FIG. 9(e).

After the reset of the transformer core is completed at t=T₄, the voltages across all transformer windings become zero. As a result, the voltage across switch SW becomes equal to the voltage V_(B) of bulk capacitor C_(B), as shown in FIG. 9(b). At the same time, a part of magnetizing current i_(M) which is negative at t=T₄ starts flowing through secondary winding N_(S) and rectifier D_(F). Since the voltage across the transformer windings is zero, i_(M) stays constant until the next switching cycle is initiated at t=T₅. From FIG. 8(e), applying Ampere's law for the final time, secondary winding current i_(S) during the T₄ -T₅ ! interval is given by ##EQU32##

Also, during the T₄ -T₅ ! interval, shown in FIG. 8(e), i_(LF) continues to decrease with the downslope given in Eq.37, while i_(LB) continues to decrease with a smaller downslope given by ##EQU33## as shown in FIG. 9(c).

From the preceding analysis of the proposed S⁴ ICS circuit operating with L_(B) in CCM, it can be seen that the leakage inductances L_(1k1) and L_(1k2) of auxiliary windings N₁ and N₂ only play significant roles during the commutation periods T₀ -T₁ ! and T₂ -T₃ !. Namely, assuming negligible leakage inductances, i.e., L_(1k1) =L_(1k2) =0 in FIG. 8, the volt-second-product balance of L_(B) during the on and off times is given by ##EQU34## where Λ_(ON) and Λ_(OFF) are the volt-second products of L_(B) during the on and off times, respectively, and T_(rset) is the reset time of the transformer core, indicated in FIG. 9(b).

Since, for the fast output-voltage control, the duty cycle of switch SW is constant over a half of a line period (see Eq.15), T_(ON) and T_(OFF) are also constant. Therefore, as rectified-line voltage v_(in) ^(rec) increases towards its peak, the volt-second-product during the on time, Λ_(ON), increases, while the volt-second-product during the off time, Λ_(OFF), decreases. As a result, a volt-second-product (flux) balance of the L_(B) core cannot be maintained. the resulting large imbalance eventually leads to the saturation of the L_(B) core. To maintain the required volt-second-product balance, it is necessary to proportionally reduce Λ_(ON) and proportionally increase Λ_(OFF) as v_(in) ^(rec) increases. The desired reduction of Λ_(ON) and increase of Λ_(OFF) in the proposed circuit in FIG. 4 is brought about by leakage inductances L_(1k1) and L_(1k2) Namely, the volt-second-product balance which takes into account the leakage-inductance effect, is ##EQU35## where V_(Y) ^(ON) and V_(Y) ^(OFF) are the voltages of node Y in FIG. 8(a) given in Eqs. 21 and 33, respectively, while ΔT_(com) ^(ON) and ΔT_(com) ^(OFF) are the commutation periods T₀ -T₁ ! and T₂ respectively, as indicated in FIG. 9(b).

Eq. 41 can be rewritten as ##EQU36##

As it can be seen comparing Eqs. 40 and 42, leakage inductances L_(1k1) and L_(1k2) decrease Λ_(ON) for ##EQU37## and increase Λ_(OFF) for ##EQU38## where ΔΛ_(ON) and ΔΛ_(OFF) are the hatched areas in FIG. 9(b).

Because, according to Eqs. 20 and 29, the slopes of currents i₁ and i₂ are constant, commutation times ΔT_(com) ^(ON) and ΔT_(com) ^(OFF) are proportional to the instantaneous values of i_(LB) at the moment switch SW is closed and open, respectively. As a result, |ΔΛ_(ON) | and |ΔΛ_(OFF) | (hatched areas in FIG. 9(b)) increase as the line voltage increases toward its peak because i_(LB) increases, as illustrated in FIG. 10. Therefore, with properly selected leakage inductances L_(1k1) and L_(1k2), the volt-second-product balance on the L_(B) core can be maintained during a half of a line period even with a constant duty cycle of switch SW. To obtain the desirable results, the total leakage inductance L_(1k) =L_(1k1) +L_(1k2) should be typically 10% to 20% of the L_(B) inductance.

Finally, the number of turns of the transformer reset winding N_(R) is determined by the required volt-second-product balance at maximum duty cycle. Generally, at maximum duty cycle, the volt-second-product balance requires ##EQU39## where T_(ON) is the switch on-time, T_(OFF) is the off time of the switch, and V_(R) is the reset voltage. If the maximum duty cycle D_(max) =50% (T_(ON) =T_(OFF)), which occurs at low line and full load, is assumed, then from Eq. 45 follows that

    N.sub.R =N.sub.P                                           (46)

because T_(ON) =T_(OFF). As a result, the voltage across switch SW during off time is ##EQU40## The maximum stress occurs at high line when V_(B) is maximum. Therefore, for universal line-voltage-range applications, the required rating of the switch is in the 800- to 900-V range. The circuit proposed in this invention can be used with a lower voltage-rated switch (e.g. 600 V) in universal line-voltage applications if a different transformer resetting technique as, for example, RCD-clamp or active-clamp technique is used.

It should be noted that the concept explained in this invention can be extended to any other single- or multiple-switch topology. FIG. 11 shows the implementation with the flyback topology. As can be seen from FIG. 11, this implementation does not require a separate reset winding because the transformer reset is done by the output voltage through the secondary-winding. Also, it should be noted that in the flyback implementation a direct energy transfer from the input to the output occurs during the off time.

To verify the operation and performance of the proposed S⁴ ICS technique for both the DCM and CCM operation of L_(B), a 100-W/5-V, universal line-voltage range (90-265 Vac), forward converter S⁴ ICS shows in FIG. 4 was built. The following components were used for the implementation of the circuit with L_(B) operating in DCM C_(in) -1 μF; D₁, D₂, & D_(R) -BYM26E; D_(F) & D_(FW) -IR40CPQ045; SW-IXTK21N100; C_(B) -330 μF/450 V; L_(F) -1.4 μH; C_(F) -3×2200 μF, L_(B) -58 μH, and T₁ -EER35 core with N_(P) =N_(R) '=48 turns, N₁ =20 turns, N₂ =26 turns, and N_(S) =4 turns, leakage inductance L_(1k1) +L_(1k2) ≈4.6 μH. For the implementation with L_(B) operating in CCM, except for boost inductor L_(B) and transformer T₁ windings, all other components were the same as for the DCM operation. In the CCM implementation, L_(B) =240 μH and T₁ with N_(P) =N_(R) =48 turns, N₁ =18 turns, N₂ =12 turns, and N_(S) =4 turns, and L_(1k1) +Lp_(1k2) ≈50 μH were taken. In both implementations the same, low-cost, current-mode, PWM IC controller (UC3845) was used to implement a fast output-voltage feedback control. The switching frequencies of both implementations were 75 kHz.

FIGS. 12 and 13 show the typical line voltage and current waveforms of the built converter for the DCM and CCM implementations, respectively. Table 1 summarizes the power-factor, total-harmonic-distortion (THD), bulk-capacitor-voltage (V_(B)), and efficiency measurements for the two implementations. As it can be seen from the oscillograms and Table 1, both implementations work with a high power factor and low THD, while keeping bulk-capacitor voltage V_(B) below 450 Vdc. However, the efficiency of the CCM implementation is slightly higher. Also, it should be noted that there is a trade-off between the power factor and maximum V_(B). The DCM implementation can achieve a higher power factor (>0.9), but the maximum V_(B) is slightly over 400 V. The CCM implementation has a lower power factor (<0.9), but also the maximum voltage on V_(B) always stays below 400 V.

                  TABLE 1                                                          ______________________________________                                         Measured Performance Comparisons Between                                       The DCM and CCM Implementations                                                (V.sub.0 = 5 V, I.sub.0 = 20 A, fs = 74 kHz)                                   DCM                 CCM                                                        V.sub.in  V!                                                                         PF      THD    V.sub.B  V!                                                                          η %!                                                                            PF   THD  V.sub.B  V!                                                                          η %!                       ______________________________________                                         90    0.927   37.1   149   74.9 0.895                                                                               43.7 145   76.0                           100   0.927   37.2   158   75.5 0.894                                                                               46.3 150   77.5                           120   0.927   37.9   191   75.9 0.889                                                                               47.9 179   78.5                           220   0.919   38.6   349   74.2 0.891                                                                               45.6 318   78.2                           230   0.919   38.7   358   74.0 0.892                                                                               44.8 333   78.1                           265   0.916   37.8   409   73.4 0.889                                                                               43.0 381   77.2                           ______________________________________                                    

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. 

We claim:
 1. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation, comprising in combination:rectified ac voltage means; an energy storage inductor connected to said rectified ac means; an energy storage capacitor; means coupling said energy storage inductor and energy storage capacitor to provide energy transfer between said capacitor and said inductor; an isolating transformer having a primary winding and a secondary winding; means coupling said secondary winding to a load; switch means for periodically coupling and for periodically decoupling said primary winding to and from said energy storage capacitor; said isolating transformer having an auxiliary winding coupled to said energy storage inductor; and said auxiliary winding generating a voltage in opposition to said rectified ac voltage means when said switch means periodically couples said primary winding to said energy storage capacitor, thereby reducing the volt-time product across said energy storage inductor when said switch means periodically couples said primary winding to said energy storage capacitor.
 2. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation, comprising in combination:rectified ac voltage means; an energy storage inductor connected to said rectified ac voltage means; an energy storage capacitor; means coupling said energy storage inductor and energy storage capacitor to provide energy transfer between said capacitor and said inductor; an isolating transformer having a primary winding and a secondary winding; means coupling said secondary winding to a load; switch means for periodically coupling and for periodically decoupling said primary winding to and from said energy storage capacitor; said isolating transformer having a first auxiliary winding coupled to said inductor; said first auxiliary winding generating a voltage in opposition to said rectified ac voltage when said switch means periodically couples said primary winding to said energy storage capacitor thereby reducing the volt-time product across said inductor when said switch means periodically couples said primary winding to said energy storage capacitor; said isolation transformer having a second auxiliary winding coupled to said energy storage inductor and energy storage capacitor; and said second auxiliary winding generating a voltage in opposition to said rectified ac voltage when said switch means periodically decouples said primary winding from said energy storage capacitor, thereby reducing the voltage of the energy storage capacitor required for resetting said inductor when said switch means periodically decouples said primary winding from said energy storage capacitor.
 3. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation as in claim 1 wherein said auxiliary winding couples energy from said rectified ac voltage means to said load when said switch means periodically couples said primary winding to said energy storage capacitor.
 4. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation as in claim 2 wherein said first auxiliary winding couples energy from said rectified ac voltage means to said load when said switch means periodically couples said primary winding to said energy storage capacitor.
 5. An isolated power-supply with input current shaping and output voltage regulation, comprising in combination:rectified ac voltage means; an energy storage inductor connected to said rectified ac voltage means; an energy storage capacitor; means coupling said energy storage inductor and energy storage capacitor to provide energy transfer between said capacitor and said inductor; an isolating transformer having a primary winding and a secondary winding; means coupling said secondary winding to a load; switch means for periodically coupling and for periodically decoupling said primary winding to and from said energy storage capacitor; said isolating transformer having an auxiliary winding coupled to said energy storage inductor; and said auxiliary winding generating a voltage in opposition to said rectified ac voltage when said switch means periodically couples said primary winding to said energy storage capacitor, thereby reducing the volt-time product across said inductor when said switch means periodically couples said primary winding to said energy storage capacitor.
 6. An isolated power-supply with input current shaping and output voltage regulation as in claim 5 wherein said energy storage inductor operates in a continuous conduction mode.
 7. An isolated power-supply with input current shaping and output voltage regulation as in claim 5 wherein said energy storage inductor operates in a discontinuous conduction mode.
 8. An isolated power-supply with input current shaping and output voltage regulation, comprising in combination:rectified ac voltage means; an energy storage inductor connected to said rectified ac voltage means; an energy storage capacitor; means coupling said energy storage inductor and energy storage capacitor to provide energy transfer between said capacitor and said inductor; an isolating transformer having a primary winding and a secondary winding; means coupling said secondary winding to a load; switch means for periodically coupling and for periodically decoupling said primary winding to and from said energy storage capacitor; said isolating transformer having a first auxiliary winding coupled to said inductor; said first auxiliary winding generating a voltage in opposition to said rectified ac voltage when said switch means periodically couples said primary winding to said energy storage capacitor, thereby reducing the volt-time product across said inductor when said switch means periodically couples said primary winding to said energy storage capacitor; said isolation transformer having a second auxiliary winding coupled to said energy storage inductor and energy storage capacitor; and said second auxiliary winding generating a voltage in opposition to said rectified ac voltage means when said switch means periodically decouples said primary winding from said energy storage capacitor, thereby reducing the voltage of the said energy storage capacitor required for resetting said inductor when said switch means periodically decouples said primary winding from said energy storage capacitor.
 9. An isolated power-supply with input current shaping and output voltage regulation as in claim 8 wherein said energy storage inductor operates in a continuous conduction mode.
 10. An isolated power-supply with input current shaping and output voltage regulation as in claim 8 wherein said energy storage inductor operates in a discontinuous conduction mode.
 11. An isolated power-supply with input current shaping and output voltage regulation as in claim 5 wherein said auxiliary winding couples energy from said rectified ac voltage means to said load when said switch means periodically couples said primary winding to said energy storage capacitor.
 12. An isolated power-supply with input current shaping and output voltage regulation as in claim 8 wherein said first auxiliary winding couples energy from said rectified ac voltage means to said load when said switch means periodically couples said primary winding to said energy storage capacitor.
 13. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation as in claim 1 wherein said energy storage inductor operates in a continuous conduction mode.
 14. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation as in claim 1 wherein said energy storage inductor operates in a discontinuous conduction mode.
 15. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation as in claim 2 wherein said energy storage inductor operates in a continuous conduction mode.
 16. A single-stage, single-switch, isolated power-supply with input current shaping and output voltage regulation as in claim 2 wherein said energy storage inductor operates in a discontinuous conduction mode. 